The invention relates to a method for synchronization of radio stations, a device or a radio bus system to execute such a method, as well as uses thereof
With microwave systems it is usual to not analyze detected high-frequency signals directly but in relation to a comparison signal. Usually mixers or demodulators are used with which a detected signal is reduced with a comparison signal into a mostly lower-frequency band. It is especially advantageous if the comparison signal exhibits a time and frequency relationship to the detected signal which is as exact as possible. The more exact this relationship is, the more interference-free and the more easily the information contained in the detected signal can be deduced. If the signal of interest is sent by a transmitter station and received and analyzed in a geographically distant receiver station using the method described, this desired time and frequency relationship is not provided just like that, since both signals, that is the send signal generated in the transmitter station and the comparison signal generated in the receiver station, originate from different sources or have been derived from different sources.
It is thus of general interest to couple the sources in the transmitter and receiver station to one another in some manner. There are a variety of methods and arrangements usually employed for this purpose. A simple frequency relationship can be implemented by using oscillators with high frequency stability in the transmitter and in the receiver.
However, temperature or ageing drifts for example mean that an unknown residual frequency offset always remains here. More expensive arrangements have means which are suitable for determining the residual frequency offset and/or the phase offset. Based on the deviation values determined, the comparison source can then be controlled or adjusted, for example. A wide variety of frequency and phase regulation loops are used for this purpose. These methods are generally very expensive and susceptible to interference, especially if the source of the send source to which adjustment is to be made is not the only source of transmitter signals in the environment of the receiver station.
If a residual frequency offset exists, the phases of the two sources cannot be in a fixed relationship, in which case the phases correspond in principle to a time variable. If, for example, a time mark is sent by the transmitter station and detected by the receiver station, the two time references only match for a short time and then, depending on the residual frequency offset, diverge from each other more or less quickly since the timing of the “clocks” does not run at precisely the same speed. Furthermore it is only possible with difficulty to transmit very exact time marks since the steepness of the signal edges cannot be at just any angle since the allowed bandwidth for radio systems is subject to statutory restrictions.
Thus the following problems frequently occur with radio systems:
With radio systems operating in time division multiplexing mode a transmitter station sends a signal to a receiver station which for its part responds to this signal after an agreed time interval. If the “clock” in the receiver station is not running exactly synchronously with the “clock” in the transmitter station, the time at which the transmitter station responds is never known exactly. This prevents the distance between the transmitter station and the receiver station being able to be determined on the basis of the delay time of the signals transmitted for example. It also makes it difficult, especially with systems operating over a very wide bandwidth, to demodulate or extract information from the receive signals.
With time-synchronous complex radio bus systems the functions partly involve transmission of one or more items of information from a master station to different slave receiver stations. If this information includes handling, instructions for example, then with a few applications it is desirable for these instructed actions to execute time-synchronously, e.g. if a number of processing units for which the execution sequences are very critically coupled with regard to timing. For the reasons given above such time synchronism is generally not provided by radio bus systems. On the one hand, the instructions to the individual slave receiver stations are mostly sent sequentially and, on the other hand, the instructed actions often take a relatively long time. That is, the “clocks” in all radio stations must be synchronized very exactly to be able to operate these types of application. A synchronization with the normal DCF-77 radio clock, for example, is frequently much too imprecise and GPS-based clocks (GPS: Global Positioning System) are frequently too expensive and also still too imprecise in some cases.